In 1907 the crowd at a county fair accurately estimated the weight of an ox. The median estimate of the crowd was more accurate than estimates from farmers and other experts. This startling observation demonstrates the wisdom of the crowds or communities in specific contexts. Crowdsourcing is the process of taking a task performed by an individual and outsourcing it to a large group in the form of a contest or open call. For example, Wikipedia solicits content and editing for its online encyclopedia. Applied to HIV testing and linkage campaigns, crowdsourcing offers an opportunity to better understand the needs and preferences of hidden key populations such as men who have sex with men (MSM), including those who have high risk behaviors but do not utilize existing health services. Online contests can generate novel content, visual images, and distribution networks for HIV testing and linkage campaigns. Another HIV tool originating from the private sector, social marketing can also enhance HIV campaigns. Social marketing is the systematic application of commercial marketing concepts to the analysis, planning, execution, and evaluation of programs. Social marketing brings together professional marketing and public health expertise in order to more effectively package and deliver HIV testing and linkage services. The major distinction between crowdsourcing and social marketing approaches in designing HIV campaigns is that crowdsourcing is largely driven by community engagement and developed bottom-up whereas social marketing is driven by public health specialists and developed top-down. Our research will focus on young (18-29yo) Chinese MSM who are highly active participants in online forums, but whose power has not been tapped for promoting HIV testing and linkage to care. This subset of men has a high HIV incidence, yet remains largely untested and unengaged. We hypothesize that a crowdsourced campaign will have a higher impact on HIV testing and linkage to care because of greater campaign engagement. Our multi-disciplinary research team brings together crowdsourcing and social marketing experience, a long term history of working with and on behalf of MSM in China, and a rich local infrastructure for HIV research, to investigate the following aims: (1) Develop a crowdsourced intervention and a social marketing intervention to enhance HIV testing and linkage among young adult MSM; (2) Compare the effectiveness of a crowdsourced intervention and a social marketing intervention on HIV testing and linkage using a multisite, quasi-experimental trial; and (3) Estimate the impact of crowdsourced and social marketing interventions among young MSM on HIV transmission in four cities using modeling. Implementing partners at Social Entrepreneurship for Sexual Health (SESH) Global, a non-governmental organization, have developed and piloted crowdsourcing methods to promote HIV testing in China. Long standing in-country research projects and infrastructure at UNC Project-China increase the likelihood of success. This research will contribute to enhanced HIV campaigns and ultimately lead to decreased HIV acquisition and transmission.